1. Field of the Invention
The present invention relates to a radio terminal equipment such as a portable telephone or the like that is provided with an oscillator for use, for example, in semiconductor integrated circuits, more particularly, an oscillator used in fields in which accurate frequencies are needed and, in some cases, certain frequency shifts need to be dealt with.
2. Description of the Prior Art
Conventionally, an oscillator for use in radio terminal equipment of this kind is made up of an oscillation inverter built in a semiconductor integrated circuit, an external or built-in resistor and an external capacitance and crystal oscillator; and its oscillation frequency is fixed.
FIG. 1 is a circuit diagram depicting the configuration of the oscillator used in the traditional radio terminal equipment. Reference numeral 101 denotes an oscillation inverter built in a semiconductor integrated circuit forming radio terminal equipment 104; reference numeral 102 denotes an output pin of the semiconductor integrated circuit to which an output terminal of the inverter 101 is connected; and reference numeral 103 denotes an input pin of the semiconductor integrated circuit to which an input terminal of the inverter 101 is connected.
Reference numeral 105 denotes a feedback resistor that forms part of an oscillator identified generally by 160, the feedback resistor 105 being connected to the output and input pins 102 and 103; reference numeral 106 denotes a crystal oscillator connected in parallel to the feedback resistor 105; and reference numerals 107 and 108 denotes capacitors (capacitances), which are connected between the output pin 102 and a ground level position 109 and between the input pin 103 and the ground level position 109, respectively.
Turning next to FIGS. 2, 3 and 4, the operation of the radio terminal equipment will be described. FIG. 2 is an explanatory diagram showing the mode of communication of the radio terminal equipment, and FIGS. 3 and 4 are frame timing diagrams of the radio system. A base station 111 has an oscillator that oscillates at a fixed frequency fO (Hz), and radio terminal equipment 112 that is a mobile station has the oscillator 160.
Since the oscillator of the radio terminal equipment 112 as a mobile station has such a configuration as mentioned above, it oscillates at a frequency independent of the fixed oscillation frequency of the base station 111. It is desirable that the base station and the mobile station use a system clock of exactly the same oscillation frequency; in practice, however, such various factors as listed below cause a frequency shift in their oscillation.
a. Variations in the performance of crystal oscillators used in the oscillators of the base and mobile stations lead to the variations in oscillation frequency.
b. The oscillation frequency has temperature dependence; the oscillation frequency decreases with an increase in temperature and vice versa. Hence, if the base and mobile stations are in different temperature environments, a considerable difference may sometimes arise between their oscillation frequencies.
c. When the mobile station is moving at high speed, its oscillation frequency as observed from the base station is high or low due to the Doppler effect, depending on whether the mobile station is approaching or going away from the base station.
The oscillation frequencies of the base and mobile stations are caused to differ by various other factors as well.
Let it be assumed that the oscillator 160 of radio terminal equipment 112, used as a first mobile station, is oscillating at a frequency f1 (Hz) a little lower than the oscillation frequency f0 (Hz) of the base station 111 and that the oscillator of radio terminal equipment 112, used as a second mobile station, is oscillating at a frequency f2 (Hz) a little higher than the oscillation frequency f0 (Hz) of the base station 111. When the oscillation frequencies of the base and mobile stations differ as mentioned above, frame timing based on the frequency difference will differ as depicted in FIG. 3. That is, the frame timing of the radio terminal equipment 112 as the first mobile station lags behind the frame timing of the base station 111, whereas the frame timing of the second mobile station 112 leads the frame timing of the base station 111. On this account, the difference in frame timing, even if very small, will accumulate as indicated by W1 and W2 during a long period of time of use; hence, the frame timing of the first and second mobile stations keeps on deviating from the frame timing of the base station 111, resulting in frame-matched, correct transmission and reception becoming impossible.
As a solution to such a problem as referred to above, there has been proposed a method that uses DPLL (Digital Phase-Locked Loop) techniques to correct for the timing deviation by a very small amount of time on a frame-wise basis. In this instance, when the frame-by-frame timing deviations of the first and second mobile stations 112 are smaller in absolute value than a correction value Dt that is a small amount of time, their frame timing follows that of the base station 111. When the absolute values of the timing deviations for each frame exceeds the correction value Dt, the timing deviations will accumulate to such relatively large amounts of time t1 and t2 as shown in FIG. 4. Accordingly, the frame timing of the first and second mobile stations 112 goes on departing from the frame timing of the base station 111; as is the case with the above, no frame-matched, correct transmission and reception are possible and no correct data reception can be expected, either. Another problem is the generation of psychoacoustically uncomfortable noise.